Milk Foams for Coffees: To Foam or Not to Foam, That is the Question

Background briefing: Steam frothing of milk

One of the most common complaints about milk is from coffee shop proprietors whose milk will not froth well when making a cappuccino or other coffee/milk drinks where the milk is foamed.

When milk is frothed for making cappuccino coffee, steam from a steam generator is injected, with air, into the milk to create foam and to heat the milk to near boiling. Both the temperature of the milk and the volume and stability of the foam are important for a good cappuccino. Ideally, some believe that the volume of the frothed-up milk should be more than twice the original volume of the milk, and the foam should be stable for at least 10 minutes, or the time it takes to drink a cup of coffee. Unfortunately, milk sometimes fails to foam during the injection of steam and hence cannot be used for making cappuccino.

Problems Associated with the Farm Raw Milk

Lipolysis

The most common reason for this is the breakdown of milkfat because of lipolysis, usually before pasteurization. Lipolysis produces free fatty acids, mono-, and diglycerides. It is caused by the action of lipase in the milk, either naturally present or produced by contaminating psychrotrophic bacteria.

Mono- and diglycerides are surface-active agents, which depress the frothing capacity of the milk. The steam frothing capacity of milk decreases as the free fatty acid level, the common indicator of lipolysis increases. Lipolysis can occur spontaneously in the milk from some cows when it is cooled soon after milking, and it can be induced by physical treatments of the raw milk such as agitation. The former tends to occur when the cow's level of nutrition is low and she is in late lactation. The simultaneous occurrence of these factors in herds in addition, throughout regions, is a common cause of the problem. This can happen during a period of adverse weather conditions and/or when cows are seasonally calved.

Agitation of milk most commonly occurs at the farm when air leaks into the teat cup cluster and air and warm milk are vigorously mixed in the milking equipment. It also occurs in the factory because of air incorporation when raw milk is pumped excessively.

Mastitis

Another factor, which adversely affects steam-frothing capacity, is mastitis. It has been shown that frothing capacity decreases as somatic cell counts increase. At least part of the explanation for this is the amount of proteolysis of casein caused by the natural protease in milk, plasmin, which occurs in elevated levels in mastitis milk. There is some evidence for this since the level of the proteose peptone (fragments of casein produced by plasmin action) in milks is inversely related to frothing ability. In fact, frothing capacity is reduced when protease peptone is added to milk.

Producing the Milk Foams

The foaming capacity of milk is largely due to the whey proteins, especially lactoblobulin. This globular protein forms a film on the surface of the air bubbles in the foam. When this protein is heated, it denatures, that is unfolds, and is even more efficient in coating and stabilizing the air bubbles. Thus, one way of improving the frothing capacity of milk is to heat it and cool it before trying to froth it. This improvement in frothing is presumably due to the denaturing of the whey proteins. UHT milk whose whey proteins are around 70% denatured froths better than pasteurized milk with around 20% denaturation of the whey proteins.

Another process that improves steam frothing is homogenization. The amount of improvement increases as the pressure of homogenization is increased. Pressures around 20 MPa are very effective.

Milk solids content has a small effect on frothing capacity and might be of use in some conditions. Although a common practice, adding milk solids, often in the form of skim milk powder, can increase frothing capacity to a limited degree.

Commonly held myths about the causes of poor steam frothing:

Added water in the milk. Adding water has little effect on frothing.

Too much or too little milkfat. The milkfat content has little effect on frothing although skim milk generally gives more froth than milk containing milkfat; however, the froth in skim milk is less dense and subsides more quickly.

Due to additives in the milk. Of course, there are no additives in milk.

The milk is too fresh. Refrigerated storage of pasteurized milk for up to three days does not affect its frothing capacity.

Summary

There is insufficient knowledge of steam frothing of milk to provide a definitive understanding of all of the issues that can cause poor frothing. However, raw milk quality is a very important part of ensuring the milk will perform in the coffee shop. Managing the issues outlined will assist in providing the best cappuccino on a daily basis.